Steel mill sludge is a waste material from the steelmaking process that contains iron oxide. Steel mill sludge, or "mill sludge", differs from mill scale generally in its finer particle size and higher oil content. During the steelmaking process, and particularly during the processes in the steel plant which occur after the blast furnace, streams of waste water that carry iron oxide waste material and oil contamination are generated, and generally flow into a pit. In such pit the material settles, forming three phases, i.e., an upper oil phase or layer, a lower layer of solids and a middle layer of water. Such lower layer of solids is generally referred to as mill scale. The water layer is pumped to a waste treatment area for separation of relatively clear water from the contaminants it contains. This water clarification area may be a settling basin, a clarifier, a drag-out basin or the like, and coagulants or other water clarification treatments may be used. The material that is separated from the water is referred to as mill sludge. The solids of the mill sludge are generally inherently much finer than the mill scale which originally settled from the waste stream because it is the finer particles that tend to remain in the water layer and tend to be carried away with the water layer when it is pumped from the pit. Mill sludge also generally has a higher oil content than mill scale, because the oil tends to become more associated with the finer solids than with the coarser particles of the mill scale.
The oil contamination is generally derived from the lubricants and coolants used in the manufacture of the final products as well as lubricants from the equipment exposed to elevated temperatures during the formation of the final steel products. Such oil contamination of mill sludge limits the potential for recycling the iron oxide containing sludge back into the steelmaking process. The heat involved in the steelmaking process liberates hydrocarbons and various oxides of hydrocarbons from oily substances, creating air contamination and making it difficult to meet environmental quality standards. In addition, if the material recycled to the sinter plant (which prepares the feed to the blast furnaces) contains too much oil, operational problems such as fouled fan blades and filter bags result, in addition to the problem of excessive hydrocarbon emissions.
If the mill sludge cannot be recycled back into the steelmaking process, then its iron oxide material becomes waste rather than being a useful raw material for steelmaking.
It is thus highly desirable to reduce the oil content of mill sludge to a sufficent extent so as to render the mill sludge suitable for recycling back into the steelmaking process. It is also highly desirable to so de-oil the mill sludge by economical means so that the advantages of recycling and reduction of waste matter are not severely diminished by the costs of such de-oiling. It is an object of the present invention to provide a method of de-oiling mill sludge to a sufficient extent so as to render the mill sludge suitable for recycling back into the steelmaking process. It is also an object of the present invention to so de-oil mill sludge in a reasonably economic manner.